Algeria and Libya are the final destinations for some travelers, while others seek to reach Europe, said Laura Lungarotti, migrant assistance regional specialist in the organization's west and central Africa office in Senegal.

Most are from Niger, although others also come from central and western Africa.

Once they embark on their journeys, they face "extremely dry and difficult conditions," she said. Those who get stranded in the desert face a challenge to survive.

Part of the problem is that many would-be migrants are stopped by Algerian or Libyan authorities and are expelled back over the border into Niger's desert, Lungarotti said.

Some of those kicked out are transported directly to two transit centers run by the International Organization for Migration: outposts in the desert where the migrants can receive food, water and first aid. Others manage to make their own way there.

Despite the dangers, the migrants' numbers have been increasing since the beginning of this year, Lungarotti said. Over the past 10 months, more than 15,000 from Niger and 1,300 from other countries have reached the two transit centers: one in Arlit, closer to Algeria, and the other in Dirkou, nearer to the Libyan border.

It's unusual for so many migrants to be women and children, Lungarotti said of the latest deaths.

A recent study by the migrant organization found that the majority of migrants are men, although there are growing numbers of unaccompanied minors, she said.